Thin film solar cell with ceramic handling layer

ABSTRACT

A method for fabricating a photovoltaic (PV) cell panel wherein all PV cells are formed simultaneously on a two-dimensional array of monocrystalline silicon mother wafers affixed to a susceptor is disclosed. Porous silicon separation layers are anodized in the surfaces of the mother wafers. The porous film is then smoothed to form a suitable surface for epitaxial film growth. An epitaxial reactor is used to grow n- and p-type films forming the PV cell structures. A glass/ceramic handling layer is then formed on the PV cell structures. The PV cell structures with handling layers are then exfoliated from the mother wafer. The array of mother wafers may be reused multiple times, thereby reducing materials costs for the completed solar panels. The glass/ceramic handling layers provide structural integrity to the thin epitaxial solar cells during the separation process and subsequent handling.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/399,248 filed Mar. 6, 2009, which claims the benefit of U.S.Provisional Patent Application Ser. No. 61/068,629, filed Mar. 8, 2008.The foregoing disclosures are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to solar cells, and more particularlyto solar cells with ceramic handling layers, and methods and systems forfabricating said solar cells with ceramic handling layers.

BACKGROUND OF THE INVENTION

Silicon is the basic ingredient of many solar cell technologies rangingfrom thin film amorphous silicon solar cells to single-crystal siliconwafer-based solar cells. High efficiency solar cells start withelectronic grade polysilicon grown by chemical vapor deposition. Thepolysilicon is melted and ingots are pulled from the melt in theCzochralski process and often zone refined to produce silicon ingots orribbons of different degrees of crystal perfection. The silicon ingot isthen sliced into thin wafers by sawing or laser cutting, and solar cellsare formed on the wafers by traditional semiconductor techniques andinterconnected and packaged to last at least 25 years. Such siliconwafers are relatively expensive and thus severely impact the costs ofsolar cells in formed and packaged in the standard wafers.

Throughout the past quarter century, there have been significantinnovations in all aspects of solar cell manufacture and accompanyingreduction in cost. For example, from 1990 to 2006, wafers have decreasedin thickness from 400 μm to 200 μm. The cost of crystalline siliconstill constitutes a significant part of the overall cost, as measured bymany of the metrics used to characterize the cost of crystalline solartechnology.

A flow chart of a conventional process for manufacturing solar panels isillustrated in FIG. 1. In step 102, stock single-crystal silicon wafersare used as substrates which are cut into shapes that are approximatelysquare, often with rounded corners due to the size and shape of theoriginal wafer (200 mm diameter typically). In step 104, a photovoltaic(PV) cell structure, which is basically a diode, is fabricated on thetop surface of the wafers. The fabrication process uses epitaxial ordiffusion furnace methods to form the required thin silicon layers dopedn-type and p-type and sometimes intrinsic (i-type). The PV cells arethen assembled into an X-Y array on a substrate 106 and contacts to then-type and p-type layers are added, often by soldering tinned copperribbons to bus bars grown on the PV wafers. It has been difficult orimpossible to attain very thin solar cells using the prior art processin which individual PC cells are formed prior to assembly into the finalX-Y array needed for a completed solar panel.

The best expectation for further reductions in silicon thickness, andthereby the cost of monocrystalline silicon solar cells, is offered bytechniques in which a crystal monocrystalline silicon substrate, oftenreferred to as the base, source or mother wafer, is first treated toform a separation layer, a thin epitaxial silicon layer is thendeposited on the treated surface, and finally the deposited epitaxiallayer is separated from the source substrate to be used as thin (2-100μm) single crystal silicon solar cells. The silicon substrate isthereafter sequentially re-used to form several additional suchepitaxial layers, each producing its own solar cell. There are severalknown standard techniques for growing the separation layer, such asforming a composite porous silicon layer by anodically etching adiscontinuous oxide masking layer, or by high energy implantation ofoxygen or hydrogen to form the separation layer within mother wafer.

The epitaxial silicon layer that is formed has to be separated intactfrom the mother wafer with little damage in order to thereafterfabricate the eventual solar cell module. The separation may be precededby formation of the p-n junctions and of part or all of theinterconnections while the epitaxial layer is still attached to themother wafer. We believe that this separation process is preferably doneby “peeling” in the case where the separation layer is highly poroussilicon. Peeling implies parting of an interface starting from one edgeand continuing until complete separation occurs.

One basic process in the prior art for manufacturing epitaxial singlecrystal silicon solar modules includes the following steps: (1) forminga separation layer on a relatively thick, single crystal siliconsubstrate; (2) growing a single crystal epitaxial layer; 3) separatingthe epitaxial layer and fabricating the solar cells on the epitaxiallayer and the basic cell interconnections on the solar cells; and (4)assembling and packaging several such cells to form a solar panel.Despite the great potential of this prior art method for producingrelatively inexpensive, highly efficient solar cells, the method haseluded commercial success for at least three main reasons: (1) some ofthe unit processes are deficient and difficult to reproduce especiallyfor thin epitaxial wafers; (2) manufacturing strategy generally startsand ends with making individual wafer-size solar cells and, thereafter,assembling them into solar panels; and (3) thin cells break easily, andtheir economical processing awaits the development of new tools andequipment.

SUMMARY OF THE INVENTION

The present invention turns the prior art strategy on its head, startingwith the solar panel and rethinking the unit manufacturing steps inpanel size, starting from the surface treatment of the source wafersthrough to module encapsulation, completely eliminating the need forhandling individual thin epitaxial silicon cells. According to oneaspect of the invention, the manufacturing sequence is reversed from theconventional prior art sequence. In this aspect of the invention,multiple source wafer tiles are bonded to a support prior to theformation of individual cells, thereby enabling the use of large-scaleprocessing for solar cell fabrication, instead of the wafer-by-waferapproach previously used. This rethinking involves key innovations thatmake these unit processes robust and reliable. This approach has beenenabled by some key innovations described in this invention. Thisessentially fulfills the vision for the 2020 module, where “Cell andmodule manufacturing is based on process steps applied to whole panelsinstead of individual cells” articulated by G. Beaucarne et al. at the21^(st) European PVEC Conference in 2006. More importantly, panel sizesemiconductor processing enables a significant reduction in the cost ofsolar energy production.

One aspect of the invention includes mounting multiple wafers on asupport plate, often called a susceptor, and processing the wafers incommon. Examples of the processing include forming a separation layer,depositing silicon to form the solar cell structure, forming contacts,and separating the solar cells as a unit from the wafers.

Another aspect of the invention includes forming a separation layer inthe multiple wafers by anodizing preferably monocrystalline wafers toform a porous silicon layer. Although the anodization may be done on anassembled array of solar cell tiles, it may also be done on individualwafers.

The support plate for anodization may be generally planar or may havewindows formed there through for exposing the back side of the waferssupported on the ribs surrounding the windows. Thereby, liquidelectrolyte may be used as a backside contact.

The anodization may be performed in a serial arrangement of multiplewafer supports removably disposed and arranged between the anode andcathode in tank containing electrolytic etching solution. The supportsare sealed to the tank walls.

The anodization forms a porous silicon layer. If desired, the porositymay be graded by varying the anodization conditions during theanodization.

The porous silicon layer may be smoothed to provide a better epitaxialbase, for example, by a high temperature anneal in hydrogen, forexample, a temperature of at least 1000 C.

Silicon layers, preferably epitaxial, may be deposited by chemical vapordeposition on the porous silicon layer. Dopant precursors may beincluded in the deposition to produce a layered semiconductor structureincluding p-n junctions. The epitaxial deposition may be performed in aradiantly heated reactor with wafers mounted inside of a sleeve formedon two sides by wafer supports each mounting an array of solar cells.

Contacts may be fully or partially added to the silicon structures stillattached to the wafer supports. Additional layers may be applied tofacilitate further processing.

The fully or partially processed solar cells may be delaminated from themother wafers across the separation (porous) layer by a progressivepeeling action including clamps and a linear array of vertical actuatorsassociated with the clamps. Examples of the clamps are segmentedelectrostatic clamps or a segmented vacuum clamp.

According to further aspects of the invention, a method of fabricating asolar cell comprises: forming a stack of thin continuous epitaxial solarcell layers on a silicon wafer; forming a handling layer on the stack,wherein the handling layer includes electrical contacts to the stack;and separating the stack from the silicon wafer, wherein the stackremains attached to the handling layer. The handling layer may be aglass/ceramic material, such as a glass, glass-bonded ceramic orglass-ceramic, with a CTE which is greater than or equal to the CTE ofthe stack over the temperature range from ambient temperatures toprocessing temperatures experienced during formation of the handlinglayer. Further, before forming the handling layer on the stack aboundary layer may be formed on the stack—the boundary layer beingsilicon oxide, silicon nitride or alumina, which may act as diffusionbarriers and/or passivation layers.

According to yet further aspects of the invention a solar cellcomprises: a stack of thin single crystal solar cell layers; a handlinglayer; and a boundary layer between the stack and the handling layer,the boundary layer being attached to both the stack and the handlinglayer, the boundary layer being greater than 10 nanometers thick andparallel to the layers in the stack. The handling layer may bewaffle-shaped with an array of either square or circular apertures.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIG. 1 is a flow chart of a prior art manufacturing process for solarpanels;

FIG. 2 is a schematic side cross-sectional view of mother wafersattached to a susceptor without windows;

FIG. 3 is a schematic side cross-sectional view of mother wafersattached to a susceptor with windows;

FIG. 4 is a schematic isometric view of wafers attached to a susceptor;

FIG. 5 is a schematic isometric view of an anodic etcher capable ofsimultaneously etching multiplicities of wafers attached in a verticalorientation to each of a plurality of susceptors;

FIG. 6 is a schematic side cross-sectional view of the anodic etcher ofFIG. 5;

FIG. 7 is a schematic isometric view of an anodic etcher capable ofsimultaneously etching a number of wafers, each attached in a verticalorientation to a support frame;

FIG. 8 is a schematic isometric view of a wafer sleeve comprising twosusceptors, each with a multiplicity of wafers attached thereto;

FIG. 9 is a schematic side cross-sectional view of two mother wafersattached to a susceptor with PV cell structures formed on the uppersurfaces of each mother wafer;

FIG. 10 is a schematic side cross-sectional view of the wafers andsusceptor from FIG. 9 with a glue layer and glass layer attached to theupper surfaces of the PV cell structures which will become the backsidesof the completed PV cells;

FIG. 11 is a schematic side cross-sectional view of the wafers andsusceptor from FIG. 9 with a handling layer attached to the uppersurfaces of the PV cell structures which will become the backsides ofthe completed PV cells;

FIG. 12 is a schematic side cross-sectional view of the wafers andsusceptor from FIG. 11 with a glue layer and glass layer attached to theupper surfaces of the handling layers which will become the backsides ofthe completed PV cells;

FIG. 13 is a schematic isometric view of a solar cell panel showing themetal connection strings;

FIG. 14 is a side cross-sectional view of an array of wafer tilescovered by a flexible film and clamped to a segmented electrostaticchuck prior to separation of the highly porous silicon film.Cross-section A-A is illustrated;

FIG. 15 is a side cross-sectional view of an array of wafer tilescovered by a flexible film and clamped to a segmented electrostaticchuck after the beginning of separation of the highly porous film etchedin FIG. 7;

FIG. 16 is a top view through cross-section A-A of the electrostaticchuck in FIGS. 14 and 15;

FIG. 17 is a side cross-sectional view of an array of wafer tiles notcovered by a flexible film and clamped to a segmented vacuum chuck priorto the separation of the highly porous silicon film. Cross-section B-Bis illustrated;

FIG. 18 is a side cross-sectional view of the array of wafer tiles notcovered by a flexible film and clamped to a segmented vacuum chuck afterthe beginning of separation of the highly porous silicon film;

FIG. 19 is a top view through cross-section B-B of the vacuum chuck inFIGS. 17 and 18;

FIG. 20 is a flow chart of the first part of a manufacturing process forsolar panels in a first embodiment of the present invention;

FIG. 21 is a flow chart of the first part of a manufacturing process forsolar panels in a second embodiment of the present invention;

FIG. 22 is a flow chart of the first part of a manufacturing process forsolar panels in a third embodiment of the present invention;

FIG. 23 is a flow chart of the first part of a manufacturing process forsolar panels in a fourth embodiment of the present invention;

FIG. 24 is a flow chart of the final part of a manufacturing process forsolar panels using PV cells with backside contacts only;

FIG. 25 is a flow chart of the final part of a manufacturing process forsolar panels using PV cells with frontside and backside contacts;

FIG. 26 is top view representation of a solar cell with a waffle-shapedhandling layer with square apertures;

FIG. 27 is a top view representation of a solar cell with awaffle-shaped handling layer with circular apertures;

FIG. 28 is a cross-section, along X-X of the solar cell of FIG. 27;

FIG. 29 is a cross-sectional representation of a solar cell with ahandling layer and back side contacts;

FIG. 30 is a cross-sectional representation of a solar cell with ahandling layer and contacts on both sides; and

FIG. 31 is a cross-sectional representation of another embodiment of asolar cell with a handling layer and contacts on both sides.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described in detailwith reference to the drawings, which are provided as illustrativeexamples of the invention so as to enable those skilled in the art topractice the invention. Notably, the figures and examples below are notmeant to limit the scope of the present invention to a singleembodiment, but other embodiments are possible by way of interchange ofsome or all of the described or illustrated elements. Moreover, wherecertain elements of the present invention can be partially or fullyimplemented using known components, only those portions of such knowncomponents that are necessary for an understanding of the presentinvention will be described, and detailed descriptions of other portionsof such known components will be omitted so as not to obscure theinvention. In the present specification, an embodiment showing asingular component should not be considered limiting; rather, theinvention is intended to encompass other embodiments including aplurality of the same component, and vice-versa, unless explicitlystated otherwise herein. Moreover, applicants do not intend for any termin the specification or claims to be ascribed an uncommon or specialmeaning unless explicitly set forth as such. Further, the presentinvention encompasses present and future known equivalents to the knowncomponents referred to herein by way of illustration.

Solar Panel

In one aspect of the invention, the solar panel includes an array ofthin single crystal silicon solar cells, wherein the thin single crystalwafers are epitaxially grown and processed together as an array on a‘template’. In the array, multiple relatively thick single crystalsilicon base or mother wafers are either handled singly or attached to asuitable carrier substrate or susceptor. The assembly of susceptor andarray of mother wafers will be called an ‘array template’. Aftercompletion of the cell fabrication steps, the entire solar cell array ispermanently adhered to a suitable support sheet with an appropriateadhesive and separated as one unit from the array template. The arraytemplate may then be reused to fabricate another solar cell array. Thearray template of this invention may consist of a closely tiled array ofcircular, rectangular or square, single crystal silicon wafers formed onone of the following three types of substrates: (1) a wafer tile cutfrom a conventional thin silicon wafers; (2) a wafer tile cut from athick, zone-refined, single-crystal silicon block; and (3) a wafer tilecut from a composite structure comprising the following two blockslaminated together of (a) a thick, zone-refined, single-crystal siliconblock, and (b) a thick, non-device-quality, silicon block.

The assembly onto a support substrate of multiple rectangular or squareepitaxial wafer tiles, hereinafter simply referred to as ‘source wafertiles’, unlike the usual circular wafers, allows for the densest packingof cells in the panel. The machining loss of high quality silicon has nomajor impact on the overall cost of silicon, while the close-packedarray pays great dividends because of the gain in cell density on thepanel. As described below, independent of the thickness of the wafertile, the preferred thickness for the final solar cell is in thepreferred range of 10-50 μm, separated from the upper surface of thewafer tiles in the closely tiled array, as described in section belowregarding separation.

Array Template

This section describes the formation of an array template, which is thefirst step in the large-substrate manufacturing process outlined above.Since the substrate of the template supports the wafers during epitaxialdeposition, it also serves as the susceptor for the epitaxial growthprocess.

FIG. 2 is a schematic side cross-sectional view of wafers 401 attachedto a susceptor 403, which in this embodiment may be a flat sheet orgenerally planar plate without windows. For effective subsequentprocessing steps, it is important to minimize the width of gaps 402between the wafer tiles 401. Typically, the wafers 401 are silicon andare doped to be conductive for reasons described below. The wafers 401should be monocrystalline to allow the subsequent epitaxial growth ofgenerally monocrystalline silicon. Excessively large gaps 402 may resultin undesirable particle generation during subsequent processing steps.The lower surfaces of the wafer tiles 401 are bonded to a supportsubstrate 403 to ensure that the upper surfaces of the wafer tiles areapproximately co-planar. The co-planarity involves two requirements: (1)the thicknesses of wafer tiles 401 must be uniform, and (2) thethicknesses of a bond layer 504 in FIG. 4 between the back surfaces ofthe wafer tiles 401 and the top surface of the susceptor 403 must beuniform. The desire for co-planarity arises from the desire to reduce oreliminate deposition on the exposed edges of the wafer tiles at the gaps402.

To optimize the anodic etching process used to form the porous siliconlayer (see FIGS. 5-7), it may be desirable to provide good electricalcontact not only between the electrolytic etching solution and the frontsides of the wafers but also between the etching solution or otherelectrolytic liquid and the backsides of the wafer tiles being etched.Thus, FIG. 3 shows a schematic side cross-sectional view of wafers 421attached to a susceptor 425 with windows 424 between ribs 426 of thesusceptor 425 providing good backside contact to the etching solution inaddition to separate frontside contact to the etching solution. The sameconsiderations with respect to the gaps 402 in FIG. 2 apply to gaps 422in FIG. 3. In this embodiment, the backsides 424 of the wafers 421 areopen to the etchant solution through the windows 424, which are of thesame general shape and only slightly smaller the tiles 421. That is, theribs 426 form a rectangular grid and the ribs 426 support and are sealedto the peripheries of the wafers 421.

FIG. 4 is a schematic isometric view of wafers 508 attached in atwo-dimensional array to a susceptor 502, more generally called asupport. The horizontally extending gaps 510 and vertically extendinggaps 512 between the wafers 508 should be minimized due to theconsiderations discussed above with reference to FIGS. 2 and 3. A largenumber (in the illustrated example, 12×6=72) of wafer tiles 608 areshown attached to the substrate 502.

The susceptor 502 must be fabricated from a material which is compatiblewith device processing conditions such as chemical vapor deposition,plasma etching, contact formation, and such. Appropriate materials forthe susceptor 502 may be ceramic or metal. Examples of ceramics orotherwise robust materials are alumina, aluminum nitride, siliconcarbide, silicon-impregnated silicon carbide, silicon, silicon nitride,boron nitride, boron carbide, etc. A planar susceptor needs to beelectrically conductive to the anodizing current while a windowedsusceptor, though preferably conductive, may be insulating. The wafertiles 508 can be held to the surface 504 of the susceptor 502 bymechanical clamps, by machined dove tail joints, by gravity, or simplyby a bond to the support by field assisted bonding well known in theart. The bonding is needed only once for an array template used informing a large number of solar cell arrays. Such large area devicefabrication is routinely done for large displays, and even for thin filmsolar panels.

Porous Silicon Layer Process

The next steps in the described process for manufacturing solar panelsinvolve the formation of a porous silicon separation layer. The purposeof this layer is to enable the reuse of the silicon wafer tiles in thetiled array created in FIG. 4 as described above. This reuse is possiblebecause the porous silicon release layer does not use up more thanroughly 7 microns of the wafer thickness. Since the thickness of themother wafer tiles is typically at least hundreds of microns (even forthin silicon wafers) and can be up to 10 mm or greater (for thicksilicon blocks or laminated silicon wafers or blocks), it is possible tofabricate a substantial number of solar cell arrays from a single arrayof wafer tiles. In order to use only a thin slice of the full wafer tilethickness, it is necessary to build the solar cells on top of a poroussilicon separation layer. K. V. Ravi in co-pending U.S. patentapplication Ser. Nos. 12/290,582 and 12/290,588, both filed Oct. 31,2008, incorporated herein by reference, describes the fabricationprocesses for backside contact PV cells, and frontside/backside contactPV cells, respectively, and are incorporated by reference herein. Thedescribed processes involve the formation of a porous surface layer inthe mother wafers and growth of an epitaxial layer over the porouslayer, and at least partial development of the solar cell in theepitaxial layer while still attached to the array template. After thearray of solar cells has been at least partially fabricated in theepitaxial layer, the tile array can be separated from the bulk siliconmaterial of the source wafer tile array, leaving most of the bulkmaterial remaining in the mother waters to be used in the formation ofadditional arrays of solar cells. For subsequent uses of the sourcewafer tile array to form second, third, fourth, etc., solar cell arrays,the surface of the mother wafers would be the typically rough lowersurface of the cleaved porous silicon layer formed in FIGS. 5-7, below.

We have discovered that a lapped surface on the silicon source wafer isespecially suited for ease of peeling of the epitaxial layer. Althoughwe are not bound by the theory, we believe that the residual surfacedamage in the lapped surface pre-disposes the porous layer formedthereon to be easily detached. Lapping produces a surface roughnessintermediate that produced by grinding and polishing. Lapping involvesrotating a planar surface of a disk, often metal and perhaps textured orgrooved, against the surface with typically a lapping powder beingdisposed between the disk and the surface. It typically produces an RMS(root mean square) surface roughness of 50 to 100 nm. Grinding involvesrotating the circular face of an abrasive grinding wheel against thework piece surface. It typically produces an RMS surface roughness ofgreater than 100 nm. Polishing is similar to lapping but uses a softerpolishing powder and a typically softer non-metallic polishing pad totypically produce a surface roughness of less than 50 nm. The rougherground silicon surface is rougher than a lapped surface and may enableeven easier peeling, but the rougher ground surface may lead to too manydefects in the epitaxial layer grown thereon. A polished siliconsurface, on the other hand, may be nearly free of surface flaws, but webelieve that the porous layer formed thereon will be relatively moredifficult to detach.

A schematic isometric view of an anodic etcher capable of simultaneouslyetching multiplicities of wafers attached in a vertical orientation toeach of a plurality of susceptors is shown in FIG. 5. The anodic etcher601 contains within dielectric tank walls two electrodes 604, 605preferably formed of platinum and electrically connected to a powersupply 606 by respective wires 606, 607. One or more susceptors 610,each having a multiplicity of wafer tiles 611 affixed thereto, areremovably immersed in the electro-etching solution 603, typicallyhydrofluoric acid (HF). If the windowed susceptor shown in FIG. 3 isused, then both the front and back sides of the wafer tiles 611 will beexposed to the electrolytic solution, but the wafer tiles 611 need to besealed to the ribs 425 of the windowed susceptor 426 to electricallyisolate the electrolytic solutions at the front and back. A conductivewindowed susceptor is preferred for anodization although it presentssome challenges. Alternatively, one or more holes through an otherwiseplanar susceptor for each wafer provides liquid contact to the wafers.Alternatively, the non-windowed susceptor shown in FIG. 2 may be used ifthe susceptor is electrically conductive and in good electrical contactwith the source wafers affixed thereto.

Furthermore, if the edges of the susceptors 610 form a seal against theinternal walls of the anodization tank 601 interrupting the electricalpath of the electrolytic solution and the etching solution does notextend over the top of the susceptors 610, then the susceptors 610 andtheir attached wafer 611 will essentially form electrodes in a serialarrangement for the anodic etching process and not require actualelectrical connections of the wafers or susceptors to the power supply606. The liquid backside contact is advantageous in ensuring uniformetching across the surfaces of the wafers 611. With proper bias on thepower supply 606, i.e., a positive bias on the fronts of the wafersrelative to their backs, only the front surfaces of the wafers 611 willreact with the HF solution 603 as is familiar to those skilled in theart. As mentioned above, a top surface of the electrolytic solution 603,typically hydrofluoric acid, is below the tops of the susceptors 610 toensure that each susceptor 610 and attached wafers 611 form a separateelectrode in the electro-etching circuit.

Etching a large array of silicon wafers to produce the needed porouslayer structures requires uniform anodic current distribution acrossindividual wafers, and between all wafers in the array. Further, thesilicon wafers need to be conductive to the anodization current, forexample, having an electrical conductivity in the range of 0.001 to 0.1ohm-cm, 0.02 ohm-cm being a convenient value. Either p-type or n-typesilicon wafers can be anodized. Discrete metal electrode contacts foreither the anode or cathode do not yield the desired level of uniformityin etching, leading to non-uniformities even within single wafers. Wehave observed that using the electrolyte itself as the electricalcontact to both sides of the wafers in a vertical etcher virtuallyeliminates these non-uniformities. Here, the same current density flowsthrough all the wafers in the array. The volumes between each pair ofsusceptors or between a susceptor and an electrode are essentially likethe individual cells in a serially connected battery. Such a scheme alsoallows for anodic etching of several panels in parallel in a verticalconfiguration as shown in FIGS. 3-5. This novel scheme produces uniformanodic etching on each wafer surface and the same etching uniformityacross all wafers in a panel, an important feature enabling processingat the panel level. When the wafers are attached to the susceptor orsupport substrate, the support substrate needs to be in good electricalcontact with the wafer, and should be a good electrical conductoritself. Alternatively, the susceptor may have openings or slots to allowfor the electrolyte to directly contact the wafer back sides. Theseinnovations enable very high throughputs in production.

An embodiment of an anodization tank shown in a schematic sidecross-sectional view of FIG. 6 of the anodic etcher 601 of FIG. 5 cancontain up to at least five susceptors 610, each having a multiplicityof wafers 611 affixed thereto. The manufacturing sequences described inFIGS. 20 and 21 would employ such an electro-etching arrangement.

However, for the manufacturing sequences described in FIGS. 22 and 23, adifferent anodization arrangement may be employed. An anodic etcher 621illustrated in the schematic sectioned isometric view of FIG. 7 iscapable of simultaneously etching a number of wafers 631, each attachedin a vertical orientation to a support frame 630. Note that in thiscase, although multiple wafers may be simultaneously etched, therebyimproving etching throughput, these wafers are not yet detachablyattached to a susceptor, and are, instead, attached to the supportframes 630, which form seals against the internal walls of the etchchamber 621. Either round or square wafers 631 may be attached to thesupport frames 630 mounted within etch chamber 621, which can be muchsmaller than the etch chamber 601 of FIGS. 5 and 6. Electrodes 624, 625are electrically connected to a power supply 628 though respective wires626, 627. The top surface of the etch solution 623, typicallyhydrofluoric acid, filled into the anodization tank 621 should be belowthe tops of the support frames 630 and the support frames 630 should besealed to the sidewalls of the anodization tank 621 to isolate the cellsof the serial electro-etching apparatus to ensure that each wafer formsan electrode in the etching circuit.

It may be advantageous to vary the anodization process to form a lowporosity film on the upper surfaces of wafers 611 or 631, and a higherporosity film below the low porosity film. Such a graded porosity hasthe advantage that the low porosity silicon layer may be easier tothermally smooth in the respective steps 203, 223, 243, and 263 in FIGS.20-23) prior to epitaxial growth of the n-type and p-type layers in thePV cells. The electro-etching process for generating a high porosityfilm in the wafers has different etch parameters than theelectro-etching process for generating a low porosity film, however theconfiguration of the electro-etching apparatus as shown in FIGS. 5-7 canbe used for both cases and for some types of grading one electro-etchingapparatus may serially anodize both sub-layers. The ability to modulateporosity by changing etch chemistry, etch current, or both has been animportant innovation.

Thermal Smoothing

After the electro-etching processes illustrated in FIGS. 5 and 6 toproduce the porous silicon surface layer, the wafer tile arrays, stillattached to the susceptors 610, are removed from the anodization tank601 for subsequent processing using various standard semiconductorprocesses, starting with thermal smoothing followed by epitaxialdeposition of silicon in a reactor. In the case of the anodizaton tank621 of FIG. 7, the wafers are typically remounted onto a susceptorcarrying a closely packed array of solar wafer tiles including themother wafers using the mounting methods described for susceptor 502 ofFIG. 4. The windowed susceptor of FIG. 3 may be used in the epitaxialreactor described below if the wafers are sealed to the ribs; however,radiant heating of the wafers through the windowed susceptor presentschallenges in accomplishing uniform heating. The planar susceptor ofFIG. 2 is more easily used in a radiantly heated epitaxial reactor.

The exposed surface of the anodized porous silicon layer isadvantageously smoothed to promote epitaxial growth of silicon on theporous layer. The smoothing may be performed as described in the aforecited Ser. No. 12/290,588 by annealing the mother wafer(s) in a hydrogenambient at generally atmospheric pressure for a time of about 10 minutesand a temperature of 900 C or higher. Although the thermal smoothing canbe done in conventional thermal processing oven, advantageously it isdone when the mother wafers are mounted on susceptors, which areassembled into a wafer sleeve and thermally smoothed as a group, forexample, using the radiantly heated epitaxial reactor described next.

Epitaxial Silicon Growth

The epitaxial reactor for depositing the silicon onto the conditionedsurfaces of the source wafer template array has been described in detailby Sivaramakrishnan et al. in co-pending U.S. patent application Ser.No. 12/392,448, filed Feb. 26, 2008 and incorporated herein byreference. The epitaxial reactor has been designed to rapidly andsimultaneously deposit silicon onto a large number of wafers by athermal chemical vapor deposition (CVD) process using radiant lamps. Asshown in the partially sectioned isometric view of FIG. 8, multiplewafers 720 are held on the interior surfaces of each of two susceptors706 facing each other at close distance to confine the heat and thereacting gases close to the wafer surfaces. The assembly of twosusceptors 706 and two end caps 701 forms a “wafer sleeve” with two openends and having a relatively small interior volume. The two end caps 701each have a tongue 702 which fits between the two susceptors 706,defining the spacing between the susceptors 706 at each susceptor edge.

The flow direction of the reactor gases flowing though the wafer sleevebetween its open ends is reversed frequently in what is called“cross-flow processing” to avoid gas depletion at middle regions of thesusceptor, thereby improving deposition thickness and resistivityuniformity. These features provide for excellent uniformity intemperature and reactive gas supply, ensuring highly uniform epitaxialsilicon deposition. The reactor may be equipped with two or threereaction chambers in series, the first one to preheat the susceptor, thesecond for the deposition of the epitaxial silicon, and the last one tocool the susceptor. Dopant species can be bled into the reactionchamber, as necessary, to form as grown junctions. This reactorarrangement greatly enhances the throughput of the epitaxial reactor.Important advantages of the epitaxial reactor and process of thisinvention are: (1) a large-area vertical reactor with low volume tominimize gas cost and footprint; (2) a high-growth rate (2-10 μm/min) inthe mass transport regime at temperatures exceeding 1000 C; (3) multiplewafer processing on two or more wafer susceptors that are processedsimultaneously since the gases and incandescent heating lamp array isshared for two susceptors within the wafer sleeve; (4) lamp basedheating for fast temperature cycling to enable a quick process sequence;and (5) efficient flow distribution for the silicon precursortrichlorosilane (TCS) with silicon conversion rates exceeding 50%.

In the case for processing the solar module of this invention, thesubstrates carrying the wafer arrays will constitute the susceptor. Thegas flow is advantageously aligned with the shorter dimension of thesolar array, again to minimize gas depletion effects in the center.

FIG. 9 is a schematic side cross-sectional view of two mother or sourcewafers 801 attached to a susceptor 800 with PV cell structuresepitaxially formed on the upper surfaces of each mother wafer 801 in theepitaxial reactor. Each PV cell includes at least one p-type layer 803and at least one n-type layer epitaxially formed on top of a poroussilicon layer 802 formed using the anodic etching process discussed inFIGS. 5-7. Note that if the anodic etching apparatus of FIGS. 5 and 6 isused, the porous silicon layer 802 will have been formed afterattachment of the mother wafers 801 to the susceptor 800; but if theelectro-etching apparatus of FIG. 7 is used, the porous silicon layer802 will have been formed prior to attachment of the mother wafer 801 tothe susceptor 800.

In either case, the growth process for the PV cells on the differentmother wafers 801 proceeds in parallel using the vertical epitaxialreactor described above. After growth of the p-doped layers 803 of thePV cells, an n-doped layer 804 is epitaxially deposited. The order ofthe p-type and n-type layers can be reversed if desired.

Epitaxial silicon can be deposited with as-grown p-n junctions by addingsuitable dopants during portions of the silicon deposition process, asshown in the two co-pending patent applications of K. V. Ravi (U.S.patent application Ser. Nos. 12/290,582 and 12/290,588) and theco-pending patent application of Sivaramakrishnan et al. (U.S. patentapplication Ser. No. 12/392,448). Such junctions may also be formedafter epitaxial growth by well known dopant thermal diffusion methods.The cell fabrication steps, including the contact formation methodsdescribed here are for examples only.

Contacts and Surface Layers

The susceptor and attached mother wafers are then removed from theepitaxial reactor and further processed for the formation of contactsand surface layers and separation from the template array and finalprocessing. Depending upon the type of cell structure, some of thecontact processing may be performed after the cell array has beenremoved from the template array.

Formation and patterning of the contacts are described in more detail inU.S. patent application Ser. Nos. 12/290,582 and 12/290,588. Therelatively large sizes and spacings of the contacts allows printing, forexample, by screen printing, of either the patterned contact material orof a patterned resist layer for deposition of the contact materialthrough the resist mask.

Several different cell types can be fabricated on the epitaxial layer.These include cells requiring double-sided contacts such as theconventional homo-junction cells and so-called double hetero junction(HIT) cells, or single-sided contacts such as integrated backsidecontact (IBC) cells. In the case of the double-sided cells, the contactson one side of the cell are formed on cells of the array while stillattached to the mother wafers on the susceptor while the contacts on theother opposite side are formed after the entire array has been glued toa glass layer and subsequently separated from the mother wafers. Theencapsulants used to bond the array to the backing sheet will limit theprocessing temperature for the second side metal contact formation. TheIBC cells are well suited for the solar array fabrication of thisinvention because all of the contact fingers can be fabricated on thecells prior to array detachment. In this case, the only remainingprocesses required to be performed on the front side of the array arecleaning, texturing, low temperature depositions of passivation andanti-reflection layers, after the solar array is separated from themother wafers attached to the susceptor.

The contacts can be fabricated using deposited thin films, such asTiPdAg, AITiWCu—CuSn patterned using shadow or resist masks or can beformed using screen printed silver-based pastes. The latter requirefiring at elevated temperatures to sinter the silver and to obtain goodelectrical contact and adhesion to the wafer. It is worthwhile to notehere that the infrastructure already exists to fabricate metalinterconnect patterns on large form factor substrates in the printedcircuit board and flat panel display industries.

In the embodiment of the backside contact of FIG. 9, openings in then-type layer 804 are made, enabling p-layer connections 805 to contactthe p-type layer 803 without shorting the n-type layer 804. Also,n-layer connections 806 are made to the n-layer 804. Details of aprocess sequence for backside contact wafer fabrication are provided inU.S. patent application Ser. No. 12/290,588.

Another aspect of the invention includes some module fabrication stepsto achieve simultaneous separation of the epitaxial solar array from thesource wafer template array. Prior to separation from the wafertemplate, the tops and sides of the entire array are encapsulated in asemi-rigid, that is, somewhat flexible, glue layer such as ethylenevinyl acetate, EVA, commonly used in solar module encapsulation. Theglue layer unites the cell array into a single somewhat flexible andpeelable entity. The glue layer is then used for attaching a rigidmodule support layer, such as glass or Tedlar, either before or afterarray separation.

As shown in FIG. 10, a glue layer 807 and a module support layer 808 areadded to the structure shown in FIG. 9. The glue layer 807 and modulesupport layer 808 are array-level features for module support. Note thatthe glue layer 807 not only covers the PV cells and the PV cell contacts805, 806, but may also flow down between the p-type and n-type layers803, 804 of neighboring PV cells and between neighboring mother wafers801, which are still attached to the susceptor 800. The illustrated topsurface including the glue layer 806 and module support layer 808 willbecome the backside of the complete PV cells. The glue layer 807 flowsaround the p-n junction to hermetically encapsulate the solar cell. Themodule support layer 808, which may be formed of glass, is optional andis usually different from the final strengthening layer applied afterdefoliation. If it is used, it should be relatively flexible to permitdefoliation by progressive peeling. In general, the glue layer 807 isless rigid than the soon to be described handle layer and than thesupport layer 808.

As noted earlier for this aspect of the invention, in order for theentire cell array to be successfully separated as one unit, theindividual cells may be ‘conditioned’ to easily peel or separate fromthe porous silicon layer. This conditioning starts with proper surfacepreparation of the source wafers prior to anodization assuring apreferred structure for the porous layer stack, and the overalluniformity of this stack within wafers, wafer-to-wafer, andarray-to-array. This conditioning may also involve edge grinding orlaser removal of the epitaxial layer that wraps around the edge of thesource wafers.

Another aid for easy array separation with the required high yields isto minimize the possibility of breakage of the thin epitaxial cellsduring the separation process. The polymeric glue encapsulation and therigid or semi rigid substrate backing spanning the entire array mayadequately assure this. Another way to assure that the individual cellsdo not break during separation involves forming a rigid ‘handling layer’on the individual cells, which serves as a rigid backbone for the thincells. A preferred low cost approach is to fabricate such a handlinglayer in situ from deposited or dispensed precursors on top of theepitaxial cells. If the cell contacts are made by thin film methods,this rigid handling layer is made from polymeric materials fromprecursors. Examples of suitable polymeric materials include epoxies,polyurethanes, cyanate resins, preferably filled with inert fillers suchas fumed silica, cordierite inorganic glass powders or fibers. Asphaltand similar materials may also be suitable for forming the handlinglayer. The precursors, in the form of viscous solutions, are dispensedon top of the finished cells, and cured, as necessary to set and becomerigid. When thick film silver pastes are used to form the contacts, thehandling layer is formed by fusing a suitable glass, ceramic, or cermethandling layer onto the individual cells. Here again, a preferredapproach is to deposit a slurry of the precursor powders on theepitaxial cells and sinter them at high temperatures to flow and adhereto the epitaxial cells. It is advantageous to deposit the powderprecursors on top of the screen printed silver pattern and to sinterboth together. (A more detailed discussion of ceramic handling layers isprovided below.)

The material of the handling layer materials should be chosen toconformably coat and adhere well to the cell surface, including anymetallization thereon and to have a coefficient of thermal expansion(CTE) close to that of silicon—a CTE greater than or equal to that ofsilicon over a temperature range from ambient temperatures to thehighest temperature reached during the forming of the handling layer.Examples of suitable insulating materials useful for such in situsubstrate fabrication include certain vitreous glasses (examples Pyrex,Corning glass 7070), glass and ceramic mixtures which together fire atthe required temperatures, devitrifiable glasses which sinter andcrystallize simultaneously upon firing (examples include certain lithiumalumino-silicate or magnesium alumino-silicate glasses, and mullite,3AlO₃-2SiO₂). Cermet compositions that can be used for this applicationare Si—SiC and Al—SiC. While the handling layer is intended tostrengthen the epitaxial silicon cell, its attachment, or fabricationthereto, it also pre-disposes the cells for easy separation due to thesmall, but inevitable CTE mismatch stress between the handling layer andsilicon. The cells provided with individual handles still need moduleencapsulation with semi-rigid EVA-type adhesives, to unite them into anarray before or after array separation.

An alternative process to that shown in FIG. 10 first deposits theprecursor for a handling layer on the structure of FIG. 9, followed byheating to simultaneously sinter the contacts 805, 806 and to convertthe handling layer precursor into a handling layer 809, illustrated inFIG. 11, separately covering each of the PV cells in the array. Due tosurface tension and the deposition of the handling layer precursor awayfrom the edges of the n-type layer 804, the handling layers 809 do notextend between neighboring PV cells but instead leave gaps 812 so thatthe rigid handling layer 809 does not prevent bending between cellsduring peeling. This difference should be considered because thestiffness of the handling layer, if it were to bridge the PV cells,would make exfoliation difficult as shown in FIGS. 14-19. The handlinglayer is a cell-level feature, which may be formed in situ on theepitaxial silicon to reinforce it and to dispose it towards easypeeling.

The individual cells can be tested prior to separation by using either acell-size or an array-size probe head. It should be recognized that thetesting at this stage is limited to measuring some cell electricalcharacteristics and not directly cell performance. With a database ofthese characteristics, defective cells may be identified and replacedwith single cells from storage, prior to encapsulation. Full cell andarray level testing can only be done after the array separation andsurface finishing and cell completion steps on the peeled surface.

As shown in the schematic side cross-sectional view of FIG. 12, the gluelayer 810 and the support layer 811 are attached to the upper surfacesof the handling layers 809, which will become the backsides of thecompleted PV cells. Again, as in FIG. 10, the glue layer 810 may flowdown in the gap 812 between the handling layers 809 and then between thePV cells and mother wafers 801. If the glue layer 810 flows at leastpast the solar cell layer 803, it provides side encapsulation for thesemiconductor layers and their junctions. The relative flexibility ofthe glue layer 810 does not prevent the bending between cell duringpeeling.

As illustrated in FIG. 13, the cells in the array can be electricallyconnected to each other, i.e. strung together, in the requiredconfiguration by spot soldering a tin-coated copper strip 1550 to tabs(contacts) on the cells formed as part of the cell interconnections,prior to encapsulation. A large number (72 in this example) of PV cells1558 may be been grown on top of the X-Y array of mother wafers 508 ofFIG. 4.

Separation of the Epitaxial Silicon Cell Device Layer

For single cells, several techniques are described in the prior art forseparating the epitaxial layer from the growth substrate when theseparation layer is a porous silicon layer produced by the anodicetching method.

Japanese laid-open patent application 7-302889 describes a methodbonding a second silicon wafer to the epitaxial layer, bonding plates tothe silicon wafers, and then separating the plates by force from eachother such that the epitaxial layer remains attached to the supportsubstrate.

Japanese laid-open patent application 8-21345 is similar to the abovebut performs the separation after forming the p-n solar cell junctionthereon while both the parent substrate and the support substrates arebonded to jigs with adhesive. The support substrates can be quartz ormetal.

In U.S. Pat. No. 6,258,666, Mizutani et al. use a curved surface of asupport substrate to peel the epitaxial film away from the growthsubstrate while the latter is secured on a support member by means ofvacuum chuck, electrostatic chuck, or by mechanical clamps. A flexiblepolymer film, which could be self-adhering film or tape, is bonded tothe epitaxial film, with an edge extending outwardly from the substrateserves to grip the film to initiate the peeling action. The edge of thefilm is then secured to drum-shaped, or blotter-shaped (semi-circle)support with a prescribed radius of curvature designed to peel theepitaxial film gently as the support member is rotated. Instead of usingan adhesive tape, the holding and peeling of the semiconductor film canbe, by vacuum suction, electrostatic suction, mechanical claws, andsuch.

Each of these methods is effective in separating the semiconductor filmgrown on a porous silicon layer of a source silicon substrate, but allhave the shortcomings of needing secondary silicon, or other temporarysupport substrates, as well as other deficiencies not conducive to highproductivity. Furthermore, these and other related methods are noteasily scalable to perform the simultaneous peeling and separation ofmultiple epitaxial silicon layers from a pre-arranged array of sourcesubstrates carrying such films, which can enable enormous productivityin module fabrication, as will be detailed below.

FIGS. 15 and 18 are schematic side cross-sectional views of theseparation of the highly porous silicon layers anodically etched ineither the anodization apparatus of FIGS. 5 and 6 or that of FIG. 7. Thepeeling process typically separates the porous silicon layer in each ofthe wafers into two layers: (1) a lower layer attached to the uppersurface of the mother wafer; and (2) an upper layer attached to the PVcell structure. The surfaces of these two layers will both be physicallyand optically rough due to the separation process. This opticalroughness can potentially increase the light collection efficiency ofthe solar cell since the upper layer attached to the PV cell structureforms the front surface of the final solar cell array. The separationprocess for the porous silicon film may be accomplished in a number ofother ways, as illustrated in FIGS. 14-19 below.

Peeling by Electrostatic Clamping

In one embodiment of the separation process, an array of electrodestrips, each smaller than the width of a wafer tile and electricallyisolated from each other, are placed on a flexible polymer film bondedto the epitaxial silicon layer and acts as additionally as a dielectriclayer. The flexible film may be the previously described glue layer. Forprogressive peeling, the potential is first applied between theoutermost electrode, which is placed on the edge of the flexible film,and the grounded silicon source substrate. This localizes theelectrostatic gripping to roughly the area covered by the electrode. Asthis electrode is lifted, it will apply an upward force on the epitaxialsilicon layer directly underneath. When a sufficient upward force isapplied, it will initiate peeling at the edge between the epitaxiallayer and the source substrate. At this point, the next electrode in thearray is activated and pulled up, to propagate the peel further. Thepeel having been already initiated at the edge, the force required topropagate the peel to this second region will be much less. This actionis carried out across the entire array of electrode strips to completethe peeling of the epitaxial layer as the same actions are initiated onthe electrode strip next to the electrode strips already peeled. Tolimit the upward pull of the electrodes, a mechanical stop can be placedon the electrode lift pins. To equalize the upward lift of all theelectrodes, they can be tied to a tie bar made of a suitable dielectricsuch as a structural plastics or a ceramic such as alumina.

As illustrated in the cross-sectional side view of FIG. 14, an array ofwafer tiles 3016 is encapsulated with a semi-rigid but somewhat flexibleglue layer 3012 and clamped overhead to a segmented electrostatic chuckprior to separation of the highly porous silicon films 3010 formed ineither the anodization tank of FIGS. 5 and 6 or that of FIG. 7. Thesegmented electrostatic chuck includes a plurality of clamping strips70-77 enclosed and electrically isolated by respective dielectric films80-87. The clamping strips 70-77 are juxtaposed to different ones of thewafer tiles 3016 across the semi-rigid glue layer 3012 and, as alsoshown along section line A-A in the top view of FIG. 16. Preferably, asshown in FIG. 14, multiple clamping strips are juxtaposed to each of thewafer tiles 3016 so that each tile is subjected to a gradual peelingincluding bending of the separate wafer tiles 3016. Peeling action maybe initiated from the left in the illustration of FIG. 14, thenprogressively moving to the right.

An electrostatic clamping power supply 3024, preferably a DC powersupply, is connected through a first electrical connection 3006 toeither mother wafers 3002 or a substrate support 3025, which may be thepreviously described susceptor. The substrate support 3025 may be atground potential as shown in the figure; however, another potential maybe used for proper clamping operation. The other output of the powersupply 3024 is connected through a second electrical connection 3004 toa series of switches 50-57 as shown, all but the switch 50 separatingdifferent ones of the electrostatic clamping strips 70-77. When switch50 is closed as shown, the clamping voltage is applied to the firstelectrostatic clamping strip 70 through a first electrical connection60. Similarly, when the switch 51 is also closed, the clamping voltageis applied to the second electrostatic clamping strip 71 through asecond electrical connection 61. Similarly, the clamping voltage issequentially applied to the remaining serially arranged electrostaticclamping strips 72-77 through electrical connections 62-67.

The electrostatic clamping strips 70-77 are applied to the top of theflexible film 3012 on top of multiple solar cells 3016. Porousseparation layers 3010, which may be formed in either the anodizationtank of FIGS. 5 and 6 or that of FIG. 7, separate the respective solarcells 3016 and the mother wafers 3002. Each of the electrostaticclamping strips 70-77 is attached to a mechanical actuator (not shown)capable of moving clamping strips 70-77 individually in a verticalup-down motion 3018 illustrated for the first clamping strip 70 in theside cross-sectional view of FIG. 15.

The figure shows the initiation of peeling of the solar cells 3016 fromthe substrates 3002 by separation (exfoliation) across the poroussilicon layers 3010 for the portion of the solar cell 3016 underlyingthe first electrostatic clamping strip 70, which had been activated bythe closing of by the closing of the first switch 50 At this point, theremaining electrostatic clamping strips 71-77 are not yet activated,that is, are not yet clamping the associated solar cell 3016 since theswitches 51-57 remain open. Upwards arrow 3108 represents a verticalpulling motion by a mechanical actuator (not shown) attached toelectrostatic clamping strip 70. Due to the electrostatic clampingaction between the first clamping strip 70 and the leftmost solarcell(s) in the array of solar cells 3016, when the first clamping strip70 is pulled up by the actuator, the leftmost solar cell(s) are alsopulled up as shown by a separated (peeled or exfoliated) portion 3104 ofthe porous layer 3010 and the continuous flexible layer 3012 is bentbetween the attached and detached solar cells 3016. The remainingelectrostatic clamping strips 71-77 are sequentially actuated in thesequence 71, 72, . . . 77 by the sequential closing of the switches51-57 in the sequence 51, 52, . . . 57. Further mechanical actuators(not shown) respectively attached to the clamping strips 71-77 pull upon the other clamping strips 71-77 in the sequence 71, 72, . . . 77,thereby creating a peeling action from left to right in FIG. 15 toseparate the array of solar cells 3016 from the mother wafers 3002.

It is possible to initially activate all of the clamping strips 70-77 intheir down positions and then sequentially pull them up to perform thepeeling operation across the solar cell array.

These actions of separating and lifting of the flexible dielectric film3012 along with the epitaxial solar cell layer 3016 bonded thereto canbe automated to make the operation scalable to large areas. Thelarge-scale peeling advantageously allows for simultaneous peeling of aprearranged array of source wafers with their epitaxial films.

It is to be understood that the essential point of this aspect of theinvention is the application of pulling force locally at an edge of theepitaxial film to grip and pull the edge of the film at its edge andthereafter to sequentially create similar forces and separations onadjacent areas of the film until the entire epitaxial layer is separatedfrom the source wafer at the location of the separation layer. The ideatranscends any specific means for creating these actions, such asspecific electrode arrangements, the gripping means, or of the means forlifting the gripped film and such which results in a rolling motionacross the array of solar cells.

Peeling by Vacuum Suction

In another embodiment, the entire peeling action may be accomplished byvacuum suction. The vacuum suction is applied locally through an arrayof vacuum ‘strips’ placed pressing on the film, starting from theoutermost strips and working progressively across the others to theother side. The level of vacuum is designed to separate and lift thefilm from the source substrate. As this action is translated across thefilm, the separation proceeds in a predictable manner until the entirefilm is peeled and separated from the source wafer. Here again, thesequential activation of the vacuum strips in the array can be regulatedby providing a feedback loop using fluidic devices and such, and therebymade to be reliable and reproducible for peeling epitaxial siliconlayers from porous or other separation layers of mother wafers.

A progressive vacuum chucking system is illustrated in the sidecross-sectional view of FIG. 17, in which an array of wafer tiles 3306need not be covered by a flexible film but are directly clamped to asegmented vacuum chuck prior to separation of the highly porous filmformed in either of previously described anodization tanks. Similarly tothe progressive electrostatic peeling, in this embodiment, vacuumpeeling action is initiated from the left, then moving to the right. Avacuum line 3308 connects a vacuum pump (not shown) to a manifold 3302,creating a vacuum within the manifold 3302. Multiple valve actuators3370-3377 enable the opening and closing of respective valves 3380-3387.In FIG. 17, the first valve 3380 is shown open, and the remaining valves3381-3387 are shown closed. Opening of the first valve 3380 causes avacuum to be formed within a first vacuum clamping strip 90 forming thefirst segment of the segmented vacuum clamp. A first flexible hoseconnects the first vacuum clamping strip 90 to the first valve 3380 toselectively supply vacuum to it. The other valves 3381-3387 aresimilarly connected to the vacuum clamping strips 91-97 throughrespective flexible tubes 3391-3397. FIG. 19 is a top view throughcross-section B-B of FIG. 17 of the eight vacuum clamping strips 90-97and flexible tubes 3390-3397 in FIGS. 17 and 18.

The eight vacuum clamping strips 90-97 attach directly to the array ofsolar cells 3306 of FIG. 17. Porous separation layers 3310 separate thesolar cells 3306 and mother wafers 3302 held on a support 3307. Each ofthe vacuum clamping strips 90-97 is activated by respective vacuumactuators 3370, 3371, . . . 3377 and is attached to one or moremechanical actuators (not shown) capable of moving clamping strips 90-97individually in a vertical up-down motion, as shown in FIG. 18.

In an alternative embodiment, a semi-rigid glue layer may be positionedbetween the vacuum clamping strips 90-97 and the solar cells 3306,similar to the arrangement of FIGS. 14 and 15.

The side cross-sectional view of FIG. 18 illustrates the initiation ofpeeling of the solar cells 3306 (not covered by a semi-rigid glue layerin this illustration) from the support substrate 3302 by separation of aporous layer 3310. Upwards arrow 3389 represents a vertical pullingmotion of a mechanical actuator (not shown) attached to the first vacuumclamping strip 90. Due to the vacuum clamping action between the firstvacuum clamping strip 90 and the leftmost solar cell(s) in solar cellarray 3306, when the first vacuum strip 90 is pulled up by the actuator,the leftmost solar cell(s) are also pulled up as shown by separation(peeling) 3404. The remaining vacuum clamping strips 91-97 are actuatedin the sequence 91, 92, . . . 97 by the sequential opening of othervalves 3381-3387 by the respective vacuum actuators 3371-3377 in thesequence 3381, 3382, . . . 3387. Mechanical actuators (not shown)attached to clamping strips 91-97 pull up on strips 91-97 in thesequence 91, 92, . . . 97, thereby creating a peeling action from leftto right in the figure to separate the array of solar cells 3306 fromthe support substrate 3302.

Manufacturing Sequences

Several alternative processing sequences can be used in the manufactureof solar cell assembly of the present invention. Four alternativesequences are illustrated in FIGS. 20-23 for the first part of theoverall manufacturing process and two alternative sequences areillustrated in FIGS. 24 and 25 for the last part of the manufacturingprocess. Any of the sequences described in FIGS. 20-23 may be used witheither of the sequences in FIGS. 24 and 25.

Embodiment 1 Attachment of the Wafers Prior to Formation of the PorousLayer, with the Use of Thin Film Contacts

The flow chart of FIG. 20 illustrates a first embodiment of the firstpart of a manufacturing process for solar panels. In this process, instep 201, a number of mother wafers are attached to a susceptor, asdescribed in FIGS. 2-4 above. The number of mother wafers may correspondto the full number of PV wafers in a solar panel or a fraction of thefull number. Next, in step 202, porous silicon layers are formed on thesurfaces of all the mother wafers using an anodization system such asthat shown in FIGS. 5 and 6. One or more susceptors are then mountedinto a wafer sleeve as shown in FIG. 8, which is designed to interfacewith an epitaxial reactor such as that described in U.S. patentapplication Ser. No. 12/392,448.

In step 203, the wafer sleeve comprising one or more susceptors loadedwith mother wafers is heated to thermally smooth the surfaces of theporous silicon layers. This thermal smoothing process provides asufficiently smooth and crystalline surface on which to grow the n-typeand p-type layers comprising the PV cells. In the first part of step204, epitaxial deposition processes fabricate the PV cells on top of thesmoothed porous silicon layer.

After the wafer sleeve is cooled and removed from the epitaxial reactoror diffusion furnace, the wafer sleeve is disassembled to enable, in thesecond part of step 204, contact holes to be formed to the n-type andp-type layers of the cells. In step 205, thin film contacts are thenformed on the surfaces of all the wafers attached to the respectivesusceptor.

Then, in step 206, the precursor for the handling layer is deposited onthe wafer surfaces on the sides which correspond to what will be theback sides of the completed PV cells. Next, in step 207, the handlinglayer is formed by heating the handling layer precursor materialdeposited in step 206. In a variation on this processing sequence, thehandling layer precursor of step 206 may be omitted. In step 208, metalstringers are then soldered or otherwise attached electrically to thecontacts on the wafers as shown for the structure in FIG. 13.

Arrow 209 indicates that the solar panel fabrication process continueswith the process of either FIG. 24 or 25, depending on the type of PVcells used (see below).

Embodiment 2 Attachment of the Wafers Prior to Formation of the PorousLayer with the Use of Thick Film Contacts

The flow chart of FIG. 21 illustrates a second embodiment of the firstpart of a manufacturing process for solar panels. In this process, instep 221, a number of mother wafers are attached to a susceptor, asdescribed above for FIGS. 2-4. The number of mother wafers maycorrespond to the full number of PV wafers in a solar panel or afraction of the full number. Next, in step 222, porous silicon layersare formed on the surfaces of all the mother wafers using an anodizationsystem such as that shown in FIGS. 5 and 6. One or more susceptors arethen assembled into a wafer sleeve as shown in FIG. 8, which is designedto interface with an epitaxial reactor such as that described in U.S.patent application Ser. No. 12/392,448.

In step 223, the wafer sleeve comprising one or more susceptors loadedwith mother wafers is then heated to thermally smooth the surfaces ofthe porous silicon layers. This thermal smoothing process provides asufficiently smooth and crystalline surface on which to grow the n-typeand p-type layers comprising the PV cells. In step 224, epitaxialdeposition processes are used to fabricate the PV cells on top of thesmoothed porous silicon layer and contact holes are formed to the n-typeand p-type layers of the cells.

After the wafer sleeve is cooled and removed from the epitaxial reactoror diffusion furnace, the wafer sleeve is disassembled to enable in step225 thick film contacts to be screen printed on the surfaces of all thewafers attached to the susceptors. In the process of FIG. 21, the screenprinting process deposits thick film contacts not requiring the laterattachment of metal stringers.

In step 226, the precursor for the handling layer is deposited on thewafer surfaces, on the sides which correspond to what will be the backsides of the completed PV cells. Next, in step 227, the handling layeris formed by heating the handling layer precursor material.Simultaneously, the metal contacts to the wafers are sintered to makegood ohmic contact. In a variation on this processing sequence, thehandling layer precursor may be omitted, in which case step 227corresponds solely to the sintering of the metal contacts.

Arrow 228 indicates that the solar panel fabrication process continueswith the process of either FIG. 24 or 25, depending on the type of PVcells used.

Embodiment 3 Attachment of the Wafers after the Formation of PorousLayers on Each Wafer with the Use of Thin Film Contacts

The flow chart of FIG. 21 illustrates a third embodiment of the firstpart of a manufacturing process for solar panels. In this process, instep 241, porous silicon layers are first formatted on the surfaces ofeach individual mother wafer using an anodization system such as thatshown in FIG. 7. Next, a number of mother wafers are attached to asusceptor in step 242, as described above for FIGS. 2-4. The number ofmother wafers may correspond to the full number of PV wafers in a solarpanel or a fraction of the full number. One or more susceptors are thenassembled into a wafer sleeve as shown in FIG. 8, which is designed tooperate in an epitaxial reactor. The wafer sleeve comprising one or moresusceptors loaded with mother wafers is then heated in step 243 tothermally smooth the surfaces of the porous silicon layers. This thermalsmoothing process provides a sufficiently smooth and crystalline surfaceon which to grow the n-type and p-type layers forming the PV cells. Instep 244, epitaxial deposition processes are used to fabricate the PVcells on top of the smoothed porous silicon layer, and contact holes areformed to the n-type and p-type layers of the cells.

After the wafer sleeve is cooled and removed from the epitaxial reactoror diffusion furnace, the wafer sleeve is disassembled to enable, instep 245, thin film contacts to be formed on the surfaces of all thewafers attached to the respective susceptors.

Prior to attachment of the stringers, in step 246, the precursor for thehandling layer is deposited on the wafer surfaces, on sides whichcorrespond to what will be the back sides of the completed PV cells.Next, in step 247, the handling layer is formed by heating the handlinglayer precursor material. In a variation on this processing sequence,the handling layer precursor may be omitted. In step 248, metalstringers are then soldered or otherwise attached electrically to thecontacts on the wafers as shown in FIG. 13.

Arrow 249 indicates that the solar panel fabrication process continueswith the process of either FIG. 24 or 25, depending on the type of PVcells used.

Embodiment 4 Attachment of the Wafers after the Formation of PorousLayers on Each Wafer, with the Use of Thick Film Contacts

The flow chart of FIG. 23 illustrates a fourth embodiment of the firstpart of a manufacturing process for solar panels. In this process, instep 261, porous silicon layers are first formed on the surfaces of eachindividual mother wafer using an anodization system such as that shownin FIG. 7. Next in step 262, a number of mother wafers are attached to asusceptor, as described above for FIGS. 2-4. The number of mother wafersmay correspond to the full number of PV wafers in a solar panel or afraction of the full number. One or more susceptors are then assembledinto a wafer sleeve as shown in FIG. 8, which is designed to operatewith an epitaxial reactor such as that previously described.

In step 263, the wafer sleeve comprising one or more susceptors loadedwith mother wafers is then heated to thermally smooth the surfaces ofthe porous silicon layers. This thermal smoothing process provides asufficiently smooth and crystalline surface on which to grow the n-typeand p-type layers forming the PV cells. In step 264, epitaxialdeposition processes are used to fabricate the PV cells on top of thesmoothed porous silicon layer and contact holes are formed to the n-typeand p-type layers of the cells.

After the wafer sleeve is cooled and removed from the epitaxial reactor,the wafer sleeve is disassembled to enable, in step 265, thick filmcontacts to be screen printed 265 on the surfaces of all the wafersattached to the susceptors. In the process of FIG. 23, the screenprinting process deposits thick film contacts not requiring the laterattachment of metal stringers.

In step 266, the precursor for the handling layer is deposited on thewafer surfaces, on sides which correspond to what will be the back sidesof the completed PV cells. Next, in step 267, the handling layer isformed by heating the handling layer precursor material. Simultaneously,the metal contacts to the wafers are sintered to make good ohmiccontact. In a variation on this processing sequence, the handling layerprecursor of step 266 may be omitted, in which case step 267 correspondssolely to the sintering of the metal contacts.

Arrow 268 indicates that the solar panel fabrication process continueswith the process of either FIG. 24 or 25, depending on the type of PVcells used.

Completion of Solar Panel Using Backside Contact PV Cells

After the completion of the first part of the manufacturing processes ofFIG. 20, 21, 22 or 23, one of two processes may be used for the secondpart depending on the type of partial cell completed in the first part.

The flow chart of FIG. 24 illustrates a first embodiment for the secondpart of the manufacturing processing for completing a solar panel in thecase where backside contact PV cells were fabricated in the first partof the manufacturing process. The array of wafers containing the PV cellstructures on their upper surfaces are still attached to the susceptorat this point. In step 271, a glue layer, such as EVA, is laid acrossthe full array of wafers with a glass layer on top of it. If a handlinglayer was formed in one of the manufacturing sequences illustrated inFIGS. 21-23, then the EVA layer is on top of the handling layer, and instep 272, the EVA bonds the glass layer to the top surface of thehandling layer as shown in FIG. 12. In the alternative case where thehandling layer was not formed on top of the wafers, in step 272, the EVAbonds the glass layer to the top surface of the PV cells as shown inFIG. 10.

Next, in step 273, the susceptor mounting the mother wafers and PV cellsand the EVA and glass layers is placed in an exfoliation system such asthose illustrated in FIGS. 14-19. The exfoliation process leaves aportion of the porous silicon layer remaining on the top surfaces of thePV cells which are attached to the EVA and glass layer. This remnantporous layer is removed in step 274 by conventional etch processes as isfamiliar to those skilled in the art. To enhance the light collectionefficiency of the solar panel, in step 275, the front sides of thewafers are texture etched to form a surface which, when combined withthe anti-reflective coating deposited in step 276, minimizes thereflection of light and thus improves the light collection efficiencyfor the solar panel.

Completion of Solar Panel Using Frontside and Backside Contact PV Cells

The flow chart of FIG. 24 illustrates a second embodiment for the secondpart of the manufacturing process for completing a solar panel in thecase where both emitter and collector contacts were fabricated in thefirst part of the manufacturing process. At this point, the array ofwafers containing the PV cell structures on their upper surfaces isstill attached to the susceptor. In step 281, thin film contacts aredeposited on the exposed front sides of the PV cells in the solar panelbeing fabricated. Next, in step 283, a frontside EVA layer and glasslayer are laid across the panel and then thermally bonded to the frontsides of the PV cells.

Next, in step 284, the susceptor containing the mother wafers and PVcells and the EVA and glass layers on top is placed in an exfoliationsystem such as those illustrated in FIGS. 14-19. The exfoliation processleaves a portion of the porous silicon layer remaining on the topsurfaces of the PV cells, which are attached to the EVA and glass layer.In step 285, this remnant porous layer is removed by conventional etchprocesses familiar to those skilled in the art. To enhance the lightcollection efficiency of the solar panel, in step 286, the front sidesof the wafers are texture etched to form a surface which, when combinedwith the anti-reflective coating deposited in step 287 minimizes thereflection of light and thus improves the light collection efficiencyfor the solar panel.

Finally, in step 288, backside metal is deposited to form the contactsto the backsides of the PV cells in the array. In step 289,strengthening layers are then deposited on the solar panel.

Module Configuration and Packaging

The support substrate attached with polymer adhesive film in theproceeding descriptions serves merely to provide mechanical support andenvironmental protection to the solar cell.

In one preferred embodiment, for the IBC cell type, the contacts to thecell are first made at discrete pad or via locations, by thin or thickfilm means, and the bus bars or fingers are made on a polymeric film orboard, prior to bonding to the epitaxial silicon layer with EVA-type ofglue. These bus bars are joined electrically to the appropriate pads onthe cell during the lamination process. Forming the interconnectionpatterns on the flexible polymer film or substrate, or on a large glassplate, allows for it to be made offline, enabling significant reductionsin the processing time on the source wafers, so that they can be madeavailable for the next epitaxial film growth faster. Also, defectiveconductor patterns on the film can be repaired or sorted out, prior toplacing them on the much more expensive epitaxial layers. Themetallization for the interconnections can be made from sputtered metalssuch as tin coated chromium-copper, titanium-tungsten-copper, tin-coatedcopper, such as used in the printed wiring board industry. The materialfor joining of the cell to the bus bars has to be carefully chosen toenable good reliable ohmic contacts when cold pressed on to the contactpads or vias in the epitaxial layer. Examples of such metallizationinclude silver or gold powders or flakes mixed with a polymericmaterial. The polymeric material used in the conductor pastes can be thesame as the glue material used to attach the interconnection film orsubstrate to the epitaxial layer.

One great advantage of fabricating the contact and interconnectionlayers on the flexible film is that it can carry the interconnections,not just for a single epitaxial wafer, but for an entire solar panelconsisting of several such epitaxial silicon wafers, placed in apre-arranged array configuration. In combination with the peelingmethods described above, which can be scaled to peel the filmssimultaneously from an array of source wafers, this invention mayprovide an unprecedented increase in productivity.

Glass/Ceramic Handling Layers

As described above, the handling layer is formed on the thin solar cellsprior to separation of the solar cells from the silicon substrate. Thehandling layer provides a rigid backbone for the thin solar cells,protecting the thin solar cells during separation from the mother waferand subsequent handling. An example of a handling layer 809 is shown inFIG. 11, where the thin solar cell is still attached to the siliconmother wafer 801. The handling layer is integral with the thin solarcell—it is formed on the thin solar cell and as such does not require anadhesive layer (such as a glass, metal or polymer) to attach to the thinsolar cell.

The term glass/ceramic handling layer is used herein to refer tohandling layers comprising glass, glass-bonded ceramic or glass-ceramic.Glass/ceramic handling layers are generally opaque and consequently willbe on the back surface of the thin solar cell—the side facing away fromthe sun. Glass/ceramic handling layers allow for back contacts, asdescribed in detail below, where the contacts and handling layer areboth formed on the same back side of the thin solar cell. Furthermore, aglass/ceramic handling layer on the back side of a thin solar cell maybe combined with electrical contacts on both sides of the solar cell.

The glass/ceramic handling layer may be fabricated on a silicon solarcell by an in-situ process involving applying powders of glass,ceramics, or mixtures thereof on the surface of the cell, and sinteringat elevated temperatures, the glass softening enough to slump or flow onthe solar cell surface to wet and bond to it. The powders may be appliedusing a variety of techniques, such as paste dispensing and applicationof green tape, which are described in more detail below. The generalrequirements for the glass/ceramic materials used for the handling layerare:

-   -   a) the coefficient of thermal expansion, CTE, is preferably        equal to, or slightly (no more than 25%) higher than that of        silicon, although a CTE upto 5% less than that of silicon may be        tolerated (see below);    -   b) sinters well at temperatures below the melting point of        silicon, and preferably in the temperature range of 700°        C.-1000° C.;    -   c) non-reactive with silicon at the sintering temperatures;    -   d) contains no deleterious impurities that can diffuse into and        degrade the solar cell performance;    -   e) mechanically strong to minimize thickness, resist bowing and        resist fracture due to differential thermal contraction during        formation. The strength of glass-ceramic materials is greater        than 200 MPa;    -   f) able to withstanding handling, thermal excursions and        chemical exposure involved in solar cell fabrication;    -   g) when integral back contacts are used, the handling layer is        required to be an insulator;    -   h) when using silver paste to fabricate electrical contacts, the        handling layer is preferably cofireable with the silver paste in        the temperature range of 850° C.-950° C.; and    -   i) very low cost for solar cell application.

Although matched CTEs of the handling layer and the silicon solar cellare preferred, a handling layer with a slightly higher CTE is acceptableif it is made thick enough to avoid warping. If the CTE of the handlinglayer is too high (say>25% above that of silicon), the ceramic mayfracture from the resulting high tensile stress. When the CTE of thehandling layer is higher than that of the silicon the solar cell willwarp. It is advantageous to use just the minimum handling layerthickness needed to obtain an unwarped wafer, to minimize materialcosts, and to make a bonded silicon solar cell fit in the same cellfabrication tools and equipment that are used for cells of conventionalthickness.

Furthermore, it is desirable that the handling layer not induce tensilestress in the silicon that exceeds its breaking strength, as measured atits modulus of rupture. For this reason it may be desired to keep thesilicon in compression, and to deal with camber by increasing thethickness of the handling layer, as described above. The rupturestrength of silicon varies widely with crystallographic orientation. Thewafer is lowest in strength in 110 directions and highest in 111directions. Wafers cut parallel to the (111) planes can withstand thelargest tensile stress induced by the handling layer. The stress inducedin the silicon, for a given thicknesses of the handling layer andsilicon, is proportional to the difference in CTE of the two, Δ{acuteover (α)}, and ΔT, the temperature at which the two are bonded together.If ΔT is low, which would be the case if the glass component of thehandling has a low set point, it follows that the stress induced,tensile or compressive in silicon, will be low. If the stress is tensilein silicon, and does not exceed the rupture strength of the silicon, itis still safe to use the handling layer even with its CTE<CTE ofsilicon. A combination of (a) thinner handling layer, (b) low ΔT for theglass component of the handling layer, and (c) wafers cut parallel to(111) planes, should allow the use of handling layers with CTE about 5%lower than that of silicon.

More details of the glass/ceramic materials—glasses, glass-bondedceramic and glass-ceramic—are provided below.

Glass handling layers may be formed from certain non-devitrifying glasspowders, which upon heating to temperatures above the softening point,T_(s), densify by coalescence, while also wetting and bonding tooxidized silicon surfaces. These glasses are typically reflowed attemperature in the range of 50-150° C. above their glass softeningpoints. Some examples of suitable glasses include certain borosilicateglasses with CTE close to that of silicon, such as Corning's Pyrex™,Corning-7070, or Schott's Borofloat™. However, these glasses have lowerstrength than may be desired and glasses such as soda-lime silicaglasses generally have a CTE that is too large for matching with thesilicon solar cell.

The term glass-bonded ceramic refers to materials in which a glassyphase wets and bonds ceramic particles to form a dense body. To producethe dense glass-bonded ceramic, the ceramic content is typically 40% byvolume. Glass-bonded ceramic materials are used in producing lowtemperature co-fired ceramic (LTCC) substrates which are widely used inthe semiconductor industry for chip packaging. However, glass-bondedceramic materials generally have significantly higher CTEs than silicon.Some examples of glass-bonded ceramic materials include:

-   -   a) alumina (˜40 volume %)+glass such as soda-lime silica,        Corning's Pyrex™ Corning-7070 and lead-borosilicates;    -   b) mullite, which is 2 parts Al₂O₃ combined with 3 parts SiO₂,        (˜40 Volume %)+glass such as soda-lime silica, Corning's Pyrex™,        Corning-7070 and lead-borosilicates;    -   c) cordierite (˜40 volume %)+glass such as soda-lime silica,        Corning's Pyrex™ Corning-7070 and lead-borosilicates; and    -   d) silicon carbide+lithium-aluminosilicate glass (with low        coefficient of thermal expansion).        Silicon carbide may also be used as a low CTE filler in these        glass composites.

The term glass-ceramic refers to glasses of certain ternary oxidecompositions that, when heated to elevated temperatures, sinter andcrystallize to yield dense ceramic structures. Powders of these glasscompositions form predominantly low expansion crystalline phases, suchas cordierite, beta-euryptite, spodumene, and celsian. The conversion tocrystalline state is nearly complete, yielding glass-ceramics having aCTE in the range of 3.0-3.5 ppm/° C. These glasses typically sinter inthe temperature range from 750° C.˜1000° C., allowing for co-sinteringwith thick film silver conductor inks used in solar cells. For thesereasons, glass-ceramics with desired CTE, refractoriness, and highstrength are the preferred materials for fabricating handling layers.Some examples of glasses for forming desirable glass-ceramics include:MgO—Al₂O₃—SiO₂ glasses with composition ranges given in Table 1 of U.S.Pat. No. 4,301,324 to Kumar et al., incorporated in its entirety herein;Li₂O—Al₂O₃—SiO₂ glasses such as those given in Table 2 of U.S. Pat. No.4,301,324; and Al₂O₃—SiO₂—B₂O₃ glasses such as those provided in Table3.

The CTE and the thickness of the handling layer are chosen so that thehandling layer, which is formed on the silicon solar device, induces acompressive stress in the silicon, while avoiding excessive tensilestress in the handling layer, over the temperature range from the setpoint (highest processing temperature of the handling layer on thesilicon solar cell) to ambient temperatures. In this regard, theMgO—Al₂O₃—SiO₂ glass-ceramic system is a good candidate since it coversa wide range of possible CTEs by varying the MgO content of the glass.See Kumar et al., International Journal for Hybrid Microelectronics,Vol. 14, No. 4, December 1991. Glasses of the MgO—Al₂O₃—SiO₂ system withMgO content in the range of 20%—25% were tested, and among them thecompositions that gave rise to the lowest curvature when attached tothin, epitaxial silicon solar cells of about 50 microns in thickness,were selected. The applicable compositions were further narrowed down toglasses with the three main ingredients constituting 95% by weight, inthe following ranges (in wt. %): MgO-20-25, Al₂O₃-18-24, and SiO₂ 50-55,with the remaining 5 wt. % consisting of one or more additives TiO₂,ZrO₂, B₂O₃, and P₂O₅, which modify the sintering and nucleationcharacteristics of the glass powders. Some exemplary compositions areshown in Table 1, where the compositions are given as weightpercentages.

TABLE 1 Exemplary Compositions of Glass-Ceramics for the Handling LayerWt. % MgO Al₂O₃ SiO₂ B₂O₃ ZrO₂ P₂O₅ Sample 1 22 22 52.5 2 0.5 1 Sample 220 21 55 2 0 2 Sample 3 24 19 52 5 0 0 Sample 4 24.2 21.2 50.6 2 0 2Sample 5 22 22 52.5 0.5 1.5 1.5

Although glass-ceramic materials may be preferred for fabrication ofhandling layers, fabrication of handling layers using glasses andglass-bonded ceramics still have advantages over the prior art.

Before describing the details of deposition and processing ofglass/ceramic materials on the silicon solar cells, conditioning of thesilicon surface is described. The glass/ceramic materials will notadhere to bare silicon. However, a thin boundary layer of silicon oxide,silicon nitride or alumina—at least 20 nm thick, to survive thesintering process, and typically 30 to 100 nm thick—is grown on thesilicon surface. (The thickness of a silicon oxide boundary layer aftersintering is typically 10 nm or greater.) The preferred method forgrowing the silicon oxide is thermal oxidation in wet oxygen, thepreferred method for growing the nitride is PECVD, and the preferredmethod for growing alumina of atomic layer deposition (ALD). Thesemethods are preferred based on a consideration of strong adhesion to thesilicon, good insulating properties and compatibility with semiconductorprocessing. Note that it is important for the boundary layer to survivethe formation of the handling layer, since the boundary layer can act topassivate the silicon surface which is necessary for increasing thequantum yield of the solar cell. Furthermore, the boundary layer may actas a diffusion barrier for impurities from the handling layer.

More details of the glass/ceramic deposition methods on the siliconsolar cells are provided below. Note that at this stage of the processthe silicon solar cell structures are still attached to the mother waferand may require significant further processing before they arefunctional as solar cells. For ease of deposition, glass/ceramic powdersare often mixed with binders, solvents, dispersants and/or plasticizersand are used in the following forms: green (unfired) tape; pastes orslurries; or suspensions in solvents or other fluids. The depositionmethods include:

-   -   (a) for green tape, tape attachment using standard techniques        known to those skilled in the art;    -   (b) for slurries and pastes, direct casting of slurry on the        cell and direct deposition of paste on the cell, with preferred        methods of screen printing, stencil printing and nozzle        deposition;    -   (c) injection'molding of ceramic powder;    -   (d) centrifugal sedimentation;    -   (e) electrophoretic deposition; and    -   (f) plasma-spraying (simultaneous deposition and sintering).        Of these methods, deposition from a nozzle is preferred.        Inks/pastes of the glass/ceramic powders very similar to those        used for screen printing work well for nozzle dispensing. The        paste is squeezed through an x-y programmable nozzle and        dispensed on the solar cell surface in the desired pattern.        Nozzle dispensing works well for depositing handling layers        which include openings. The non-contact nature of nozzle        dispensing is advantageous due to the fragility of the thin        epitaxial silicon solar cells. Furthermore, the nozzle        dispenser's x-y movement is programmable, and easily changed to        accommodate design changes. Multiple nozzles may be used to        advantage for speeding up the process.

Handling layers may be formed as uniform layers, or may be formed intospecific structures, such as waffle-shaped handling layers with arraysof openings. FIG. 26 shows a schematic top view of a waffle-shapedhandling layer 2600 formed on an epitaxial silicon solar cell, where thehandling layer has an array of roughly square openings. For ease ofillustration, FIG. 26 is not drawn to scale, but representativedimensions are provided. FIG. 26 shows openings 2610 in the handlinglayer, which are sized and separated to allow ease of making electricalcontact to the backside of the silicon solar cell, while maintaining amechanically strong handling layer. (Note that not all of the openingsare necessarily used for making electrical contact to the solar cell.)Openings 2610 typically have widths in the range of 500 microns to 2 mm,and the center-to-center spacing of openings is typically twice thewidth of the openings or greater. The handling layer has a border 2620around the edge of the silicon solar cell, the border 2620 having atypical width of 1 mm. Nozzle dispensing works well for depositingwaffle-shaped handling layers. The waffle-shaped handling layer mayconveniently be formed by depositing a first set of parallel ridges onthe surface of the silicon solar cell, followed by a second set ofridges perpendicular to the first set.

The waffle structure of the handling layer 2600 may be misaligned by asmall arbitrary angle, θ, with respect to the underlying cleavage planesof the single crystal epitaxial silicon layers of the solar cell. Amisalignment, in the range of 1 to 40 degrees and more typically in therange of 3 to 10 degrees, from the dominant cleavage planes is effectivein reducing breakages of the solar cells. The edges of the mothersilicon wafer will commonly be defined by (100) crystallographic planes.Consequently the edges of the epitaxial silicon solar cell layers willalso be defined by (100) planes, as shown in FIG. 26. The misalignmentof the array of openings in the waffle-shaped handling layer and the(100) planes of the underlying epitaxial silicon is defined by the angleθ.

Although a misalignment of the waffle structure of the handling layer tothe underlying cleavage planes of the solar cell is preferred, wafflestructures are fabricated which are aligned with the cleavage planes forconvenience. The probability of a solar cell failure due to cleaving ofthe silicon in the openings in the waffle structure decreases with thesize of the openings. As the size of the openings becomes smaller thanroughly 200 to 500 microns the misalignment of the waffle structure withthe silicon cleavage planes becomes less important.

FIG. 27 shows a schematic top view of a waffle-shaped handling layer2700 formed on an epitaxial silicon solar cell, where the handling layerhas an array of roughly circular openings. For ease of illustration,FIG. 27 is not drawn to scale, but representative dimensions areprovided. FIG. 27 shows openings 2710 in the handling layer, which aresized and separated to allow ease of making electrical contact to thebackside of the silicon solar cell, while maintaining a mechanicallystrong handling layer. Openings 2710 typically have widths in the rangeof 500 microns to 2 mm, and the center-to-center spacing of openings istypically twice the width of the openings or greater. The handling layerhas a border 2720 around the edge of the silicon solar cell, the border2720 having a typical width of 1 mm. Nozzle dispensing works well fordepositing waffle-shaped handling layers. With circular apertures in thewaffle-shaped handling layer, the waffle structure may be misalignedwith respect to the cleavage planes of the solar cell as describedabove, although misalignment appears not to provide as much of anadvantage as it does for the handling layer with square apertures.

After depositing the glass/ceramic powders on the surface of the siliconsolar cell, this deposited layer is consolidated and adhered to thesolar cell by a high temperature firing or sintering process. Typicalprocess steps include:

-   -   (1) evaporation of solvent from the slurry/paste/tape—typically        done at a temperature between room temperature and 200° C.;    -   (2) carbonization and burning-off binders, typically done at a        temperature between 200° C. and 500° C.; and    -   (3) consolidation of the glass/ceramic material and adhesion of        the material to the solar cell; this step includes the following        changes: coalescence of glass/ceramic powders, followed by        nucleation and crystallization of the material to form a dense        handling layer.        These steps may be carried out in batch furnaces, belt furnaces        or rapid thermal processing (RTP) furnaces, for example. The        thickness of the fired handling layer is ordinarily in the range        of 5 to 1000 microns and more typically in the range of 250 to        500 microns.

Furthermore, openings may be desired in the handling layer for makingelectrical contacts to the silicon solar cell or opening windows toallow further processing of the silicon solar cell. (An example of thelatter is opening windows in the handling layer to allow the formationof distributed p-n junctions.) Openings may be formed prior todeposition of the handling layer material onto the solar cell surface,during the deposition of the handling layer, after deposition but beforefiring of the handling layer, or after firing. Some examples of suitablemethods include:

-   -   (a) pre-punching or laser-drilling holes in green tape prior to        depositing the green tape on the solar cell surface;    -   (b) for directly cast slurry or paste, openings may be formed by        the casting process itself or by laser ablation of the dried        slurry or paste layer;    -   (c) slurries and pastes may be injection molded, in the form of        layers with openings, directly onto the cell surface;    -   (d) after firing, openings may be laser drilled; and    -   (e) after firing, openings may be grit blasted.

FIG. 28 shows a cross-section 2800 along X-X in FIG. 27 of awaffle-shaped handling layer formed on a solar cell, with the additionof electrical contacts 2890. The waffle-shaped handling layer 2850 hasbeen formed on a stack of thin continuous epitaxial solar cell layers.The stack comprises: a highly-doped silicon layer 2810 and a dopedsilicon absorber layer 2820. The thickness of the stack is ordinarily inthe range of 1 to 100 microns and typically in the range of 25 to 50microns. Between the handling layer 2850 and the stack is a boundarylayer 2830. The thickness of the boundary layer is generally greaterthan 10 nanometers. The openings in the waffle-shaped handling layer2850 coincide with electrical contacts 2842 and 2844 to the stack.Electrode fingers 2890 are formed over the handling layer 2850 to makeelectrical connection to the electrical contacts. The electrode fingers2890 run perpendicular to the plane of the cross-section. Ananti-reflection coating (ARC) 2880 is formed on the textured surface2870 of the stack. Note that FIG. 28 is an example of an integrated backcontact solar cell, in which collector 2822 and emitter 2824 regions ofthe solar cell are separately contacted by contacts 2842 and 2844,respectively.

Examples of process flows for forming thin epitaxial silicon solar cellswith handling layers are provided below, for both an integrated backcontact (IBC) cell and a two sided contact cell. These examples areintended to be illustrative and represent only some of the manyprocesses that can be used. Those skilled in the art will appreciateafter reading this disclosure and following the teachings and principlesof the present invention that many variations of these processes arewithin the scope of the present invention.

FIG. 29 shows an integrated back contact (IBC) solar cell with ahandling layer according to some embodiments of the present invention. Aprocess flow for the IBC solar cell of FIG. 29 includes:

-   -   (1) form a porous silicon layer on the surface of the mother        silicon wafer;    -   (2) thermally smooth the porous silicon layer in H₂ gas at        approximately 1000° C.;    -   (3) deposit a highly doped epitaxial silicon layer 2810, such as        a p+ layer, over the porous layer;    -   (4) deposit an epitaxial silicon absorber layer 2820, such as a        p-doped layer;    -   (5) form a boundary layer 2830 on the silicon surface, for        example by controlled oxidation to form a layer generally        greater than 20 nanometers thick and typically in the range of        30 to 100 nm thick—a similar thickness of silicon nitride        (SiN_(x)) or alumina may also be used;    -   (6) open windows in the boundary layer to define the emitter        regions using laser or wet etching through a resist mask;    -   (7) diffuse phosphorous into the epitaxial silicon to create        emitter regions 2822;    -   (8) open windows in the boundary layer, through a resist mask,        to define collector regions;    -   (9) diffuse boron into the epitaxial silicon to form collector        regions 2824;    -   (10) screen print Ag—Al paste to pattern collector contacts 2842        and dry the paste pattern;    -   (11) screen print Ag paste to pattern emitter contacts 2844 and        dry the paste pattern;    -   (12) deposit glass-ceramic slurry on the printed electrodes        (with openings at electrode locations);    -   (13) co-fire the glass-ceramic slurry and the Ag—Al and Ag        pastes to form the handling layer 2850 and the cell contacts        2842 and 2844;    -   (14) exfoliate the epi-handling layer composite from the mother        wafer and send the mother wafer for forming more solar cells;    -   (15) etch front side (opposite side to handling layer) of        silicon solar cell to form a textured surface 2870;    -   (16) deposit an ARC (anti-reflection coating) 2880 on the        textured surface 2870; and    -   (17) deposit thin film aluminum, aluminum-vanadium alloy, or        chromium-copper alloy electrode fingers 2890 through a shadow        mask; this deposition may also fill the openings in the handling        layer 2850—forming metal portions 2860. (Note that the fingers        2890 run perpendicular to the plane of the figure.)        Note that the handling layer shown in FIG. 29 is an example of a        handling layer with a uniform thickness, formed from green tape,        for example, in contrast to the waffle shaped handling layer of        FIGS. 26-28. Furthermore, openings in the handling layer for        making electrical contact through the handling layer to the        emitter and collector contacts may be formed using any of the        techniques described above. Further note that the emitter and        collector regions may be formed by alternative processes, such        as epitaxially forming a n+ layer on the absorber layer 2820,        followed by etching through the n+ layer where collector areas        are needed and then diffusing boron into these areas. (Here the        remaining n+ layer is used to form emitter areas.)

FIG. 30 shows a two-sided contact solar cell with a handling layeraccording to some embodiments of the present invention. A process flowfor the two-sided contact solar cell includes:

-   -   (1) form a porous silicon layer on the surface of the mother        silicon wafer;    -   (2) thermally smooth the porous silicon layer in H₂ gas at        approximately 1000° C.;    -   (3) deposit an epitaxial silicon absorber layer 2820, such as a        p-doped layer, over the porous layer;    -   (4) deposit a highly doped epitaxial silicon layer 2825, such as        a p+ layer, over the absorber layer;    -   (5) form a boundary layer 2830 on the silicon surface, for        example by controlled oxidation to form a layer in the range of        30 to 100 nm thick—a similar thickness of silicon nitride        (SiN_(x)) or alumina may also be used;    -   (6) form a handling layer 2850 with distributed openings to the        surface of the epitaxial silicon—in this example the surface of        the highly doped epitaxial silicon layer;    -   (7) separate the epitaxial silicon, with integrated handling        layer, from the mother wafer, and send the mother wafer for        forming more solar cells;    -   (8) front side processing, including the steps of;        -   a. clean the front side surface;        -   b. etch the surface to form a textured surface 2870;        -   c. forming a phosphorus doped region 2815, which provides a            semiconductor p-n junction;        -   d. front side passivation (oxide or CVD nitride—the nitride            also acts as an ARC 2880); and    -   (9) metallization steps include:        -   a. screen print front side with Ag paste and dry;        -   b. screen print back side with Ag—Al paste, including            filling the contact holes in the handling layer and forming            finger electrodes; and        -   c. co-fire the front and back metallizations to form front            side contacts 2846, backside contacts 2844, including            contact holes filled with metal portions 2860, front side            finger electrodes 2892 and backside finger electrodes 2890.            Note that openings in the boundary layer where it is exposed            by the openings in the handling layer may be formed during            the front side clean process. Although, if necessary a            separate etch may be used for this purpose. The above            process may be modified, as will be apparent to those            skilled in the art, to form a solar cell with layers with            reversed polarities.

FIG. 31 shows another embodiment of a two-sided contact solar cell witha handling layer. The process flow for the structure of FIG. 30 may befollowed, except as noted as follows. The handling layer 2850 is awaffle-shaped handling layer, as described above. Furthermore, thebackside metallization is a single step deposition of aluminum oraluminum alloy, forming backside contacts 2844 and a common backsideelectrode 2891.

Although the solar cells of FIG. 29-31 are described as having p-typeabsorber layers 2820, solar cells may alternatively be formed withn-type absorber layers.

Although the solar cells described herein are single crystal siliconsolar cells, the teaching and principles of the present invention may beapplicable to solar cells comprising polycrystalline, microcrystallineor amorphous silicon. For example, the absorber layer 2820 mayalternatively be formed of microcrystalline or amorphous silicon.Furthermore, a polycrystalline mother wafer may be used to grow a solarcell with polycrystalline silicon epitaxial layers.

Although the solar cells described herein are silicon-based solar cells,the teaching and principles of the present invention are also applicableto solar cells comprising layers of gallium arsenide, indium galliumarsenide, gallium phosphide, gallium nitride, germanium, silicongermanium and/or silicon carbide. Furthermore, the teaching andprinciples of the present invention are also applicable to so-calledtandem solar cells, such as epitaxially grown GaN on an epitaxialsilicon layer.

Impurity gettering steps, as are well known to those skilled in the art,may also be incorporated into the process flows of the present inventionto remove impurities from the epitaxial layers.

As described above, many of the glass/ceramic materials form opaquehandling layers and thus are formed on the backside of the solarcell—the side away from the sun. The wavelength range of interest forsilicon solar cells is 300 nm to 1400 nm. Some of the handling layermaterials efficiently reflect light over this wavelength range. See, forexample, Park et al., Journal of Ceramic Processing Research, Vol. 3,No. 3, pp 153-158 (2002), which reports that cordierite glass-ceramicmaterial reflects nearly 90% of light in this range of interest.Consequently, handling layers may be used to reflect back into the solarcell most of the light which makes it through the solar cell withoutbeing absorbed. Thus handling layers may improve the light conversionefficiency of the thin film solar cells. This may provide a significantadvantage for thin film silicon solar cells with glass-ceramic handlinglayers over cells backed by glass, since glass backed cells typical lose50% of the incident light through the cell and glass.

Although the present invention has been particularly described withreference to certain embodiments thereof, it should be readily apparentto those of ordinary skill in the art that changes and modifications inthe form and details may be made without departing from the spirit andscope of the invention.

1. A method of fabricating a solar cell comprising: forming a stack ofthin continuous epitaxial solar cell layers on a silicon wafer; forminga handling layer on said stack, wherein said handling layer includeselectrical contacts to said stack; and separating said stack from saidsilicon wafer, wherein said stack remains attached to said handlinglayer.
 2. A method as in claim 1, wherein said stack has a thickness inthe range of 1 to 100 microns.
 3. A method as in claim 2, wherein saidstack has a thickness in the range of 25 to 50 microns.
 4. A method asin claim 1, wherein said stack comprises a p-type layer.
 5. A method asin claim 1, wherein said stack comprises a n-type layer.
 6. A method asin claim 1, wherein said silicon wafer includes a porous silicon surfacelayer and said separating includes applying a separating force tofracture said porous silicon layer.
 7. A method as in claim 1, whereinsaid silicon wafer is selected from the group consisting of apolycrystalline silicon wafer and a single crystal silicon wafer.
 8. Amethod as in claim 1, wherein said thin continuous epitaxial solar celllayers include a layer selected from the group consisting of asilicon-germanium layer, a silicon carbide layer, a gallium nitridelayer and a silicon layer.
 9. A method as in claim 1, wherein saidhandling layer comprises a material selected from the group consistingof glass, glass-bonded ceramic and glass-ceramic.
 10. A method as inclaim 9, wherein said handling layer comprises cordierite glass-ceramic.11. A method as in claim 9, wherein said handling layer has a thicknessin the range of 5 to 1000 microns.
 12. A method as in claim 9, whereinsaid handling layer has a thickness in the range of 250 to 500 microns.13. A method as in claim 1, wherein said handling layer has acoefficient of thermal expansion greater than or equal to thecoefficient of thermal expansion of said stack over a temperature rangefrom ambient temperatures to a maximum temperature, said maximumtemperature being the highest temperature reached during said formingsaid handling layer.
 14. A method as in claim 1, wherein said handlinglayer has a coefficient of thermal expansion in the range from 5% lessthan to 25% greater than the coefficient of thermal expansion of saidstack over a temperature range from ambient temperatures to a maximumtemperature, said maximum temperature being the highest temperaturereached during said forming said handling layer.
 15. A method as inclaim 1, wherein said forming said handling layer includes: depositing apowder; and sintering said powder.
 16. A method as in claim 15, whereinsaid powder comprises ceramic powder and glass powder.
 17. A method asin claim 15, wherein said powder is a powder paste.
 18. A method as inclaim 15, wherein said depositing is nozzle dispensing.
 19. A method asin claim 18, wherein said nozzle dispensing is dispensing from multiplenozzles simultaneously.
 20. A method as in claim 15, wherein said powderis green tape.
 21. A method as in claim 20, wherein said green tape haspreformed openings.
 22. A method as in claim 21, further comprising,before said sintering, filling said preformed openings with a metalpaste.
 23. A method as in claim 15, wherein said sintering is firing ina temperature controlled furnace.
 24. A method as in claim 15, furthercomprising, after said sintering, forming vias in said handling layer.25. A method as in claim 24, wherein said forming said vias includeslaser drilling.
 26. A method as in claim 24, further comprising, afterforming said vias, filling said vias with electrically conductivematerial.
 27. A method as in claim 1, further comprising, before saidforming said handling layer on said stack, forming a boundary layer onsaid stack.
 28. A method as in claim 27, wherein said boundary layer isselected from the group consisting of a silicon oxide layer, a siliconnitride layer and an alumina layer.
 29. A method as in claim 28, whereinsaid boundary layer has a thickness greater than 20 nanometers.
 30. Amethod as in claim 28, wherein said boundary layer has a thickness inthe range of 30 to 100 nanometers.
 31. A method as in claim 1, furthercomprising, after said separating: etching the surface of said stackopposite to said handling layer to form a textured surface; and formingan antireflection coating over said textured surface.
 32. A method as inclaim 1, further comprising, before said forming said handling layer onsaid stack, forming electrical contacts on the surface of said stack.33. A solar cell comprising: a stack of thin single crystal solar celllayers; a handling layer; and a boundary layer between said stack andsaid handling layer, said boundary layer being attached to both saidstack and said handling layer, said boundary layer being greater than 10nanometers thick and parallel to the layers in said stack.
 34. A solarcell as in claim 33, wherein said handling layer has a coefficient ofthermal expansion greater than or equal to the coefficient of thermalexpansion of said stack over a temperature range from ambienttemperatures to a maximum temperature, said maximum temperature beingthe highest temperature reached during the forming of said handlinglayer.
 35. A solar cell as in claim 33, wherein said handling layer hasa coefficient of thermal expansion in the range from 5% less than to 25%greater than the coefficient of thermal expansion of said stack over atemperature range from ambient temperatures to a maximum temperature,said maximum temperature being the highest temperature reached duringthe forming of said handling layer.
 36. A solar cell as in claim 33,wherein said handling layer reflects back into said stack visible lightpassing through said stack towards said handling layer.
 37. A solar cellas in claim 33, wherein the interface between said handling layer andsaid boundary layer reflects visible light passing through said stackback into said stack.
 38. A solar cell as in claim 33, furthercomprising electrical contacts at the interface between said stack andsaid handling layer.
 39. A solar cell as in claim 48, wherein saidhandling layer comprises metallic vias configured to provide electricalconnections to said electrical contacts.
 40. A solar cell as in claim33, wherein said handling layer has a waffle-shaped structure with afirst set of ridges and a second set of ridges, said first set of ridgesbeing perpendicular to said second sets of ridges.
 41. A solar cell asin claim 40, wherein at least one of said first and second sets ofridges are aligned at a small angle to a cleavage plane of said stack ofthin single crystal solar cell layers.
 42. A solar cell as in claim 41,wherein said cleavage plane is the 100 family of planes.
 43. A solarcell as in claim 41, wherein said small angle is in the range between 3and 10 degrees.
 44. A solar cell as in claim 33, wherein said handlinglayer has a waffle-shaped structure with an array of roughly squareapertures.
 45. A solar cell as in claim 44, wherein the square aperturesare roughly less than 500 microns on a side.
 46. A solar cell as inclaim 33, wherein said handling layer has a waffle-shaped structure withan array of circular apertures.
 47. A solar cell as in claim 33, whereinsaid boundary layer is selected from the group consisting of a siliconoxide layer, a silicon nitride layer and an alumina layer.